The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit having layers containing an organic polymer called "kerogen," which upon heating decomposes to produce hydrocarbon liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the oil shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination, surface distortion, and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598, which are incorporated herein by this reference. Such patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then fragmenting a portion of the remaining formation to form a stationary, fragmented permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen containing retort inlet mixture into the retort as a gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the gaseous combustion zone feed into the combustion zone, the combustion zone is advanced through the retort. The combustion zone is maintained at a temperature lower than the fusion temperature of oil shale, which is about 2100.degree. F., to avoid plugging of the retort, and above about 1100.degree. F. for efficient recovery of hydrocarbon products from the oil shale.
The effluent gas from the combustion zone comprises combustion gas and any gaseous portion of the combustion zone feed that does not take part in the combustion process. This effluent gas is essentially free of free oxygen and contains constituents such as oxides of carbon and sulfurous compounds. It passes through the fragmented mass in the retort on the advancing side of the combustion zone to heat oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and to a residue of solid carbonaceous material.
The liquid products and gaseous products are cooled by cooler particles in the fragmented mass in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift or shaft. An off gas containing combustion gas generated in the combustion zone, gaseous products produced in the retorting zone, gas from carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion process is also withdrawn to the surface.
It is desirable to know the locus of parts of the combustion and retorting processing zones as they advance through an in situ oil shale retort for many reasons. One reason is that by knowing the locus of such a processing zone, steps can be taken to control the orientation of the advancing side of the processing zone. It is desirable to maintain a processing zone which is flat and uniformly transverse and preferably uniformly normal to the direction of its advancement. If the combustion zone is skewed relative to its direction of advancement, there is more tendency for oxygen present in the combustion zone to oxidize hydrocarbon products produced in the retorting zone, thereby reducing hydrocarbon yield. In addition, with a skewed processing zone, more cracking of the hydrocarbon products can result. Monitoring the locus of parts of the processing zone provides information for control of the advancement of the processing zone to maintain it flat and uniformly perpendicular to the direction of its advancement to obtain high yield of hydrocarbon products.
Another reason for which it can be desirable to monitor the locus of the processing zone is to provide information so the composition of the combustion zone feed mixture can be varied with variations in the kerogen content of oil shale being retorted. Formation containing oil shale include horizontal strata or beds of varying kerogen content, including strata containing substantially no kergoen, and strata having a relatively high kerogen content such as having a Fischer assay of 80 gallons per ton. If combustion zone feed containing too high a concentration of oxygen is introduced into a region of a retort containing oil shale having a high kerogen content, oxidation of carbonaceous material in the oil shale can generate sufficient heat that fusion of the oil shale can result, thereby producing a region of the fragmented mass which cannot be penetrated by processing gases. High temperatures also can cause excessive endothermic carbonate decomposition to carbon dioxide and dilution of the off gas from the retort, thereby lowering the heating value of the off gas. Layers in the fragmented mass inherently correlate with strata in the unfragmented formation because there is little vertical mixing between strata when explosively fragmenting formation to form a fragmented permeable mass of formation particles. Therefore, samples of various strata through the retort can be taken before initiating retorting of the oil shale and assays can be conducted thereon to determine the kerogen content. Such samples can be taken from the fragmented mass, from formation before expansion, or from formation nearby the fragmented mass since little change in kerogen content of oil shale occurs over large areas of formation. Then, by monitoring the locus of the combustion zone as it advances through the retort, the composition of the combustion zone feed can be appropriately modified.
Another reason for monitoring the locus of the combustion and retorting processing zones as they advance through the retort is to monitor the performance of the retort to determine if sufficient shale oil is being produced in relation to the amount of oil shale being retorted.
Further, by monitoring the locus of the combustion and retorting processing zones, it is possible to control the advancement of these two zones through the retort at an optimum rate. The rate of advancement of the combustion and retorting processing zones through the retort can be controlled by varying the flow rate and composition of the combustion zone feed. Knowledge of the locus of the combustion and retorting processing zones allows optimization of the rate of advancement to produce hydrocarbon products of the lowest cost possible with cognizance of the overall yield, fixed costs, and variable costs of producing the hydrocarbon products.
Thus, it is desirable to provide methods for monitoring advancement of combustion and retorting processing zones through an in situ oil shale retort.